Method and apparatus for laser wavelength stabilization

ABSTRACT

In an apparatus and method for locking the wavelength of a laser, a fringe-producing optical element is disposed directly in the in the output beam from the laser. The fringe-producing optical element produces a fringe pattern in a second light beam derived from the output beam. The fringe pattern is detected by a detector unit. Signals generated by the detector unit are used to generate a laser tuning control signal that tunes the laser to a desired operating wavelength.

FIELD OF THE INVENTION

[0001] The present invention is directed generally to lasers and moreparticularly to an apparatus for monitoring and stabilizing theoperating wavelength of a laser.

BACKGROUND

[0002] The widespread introduction of wavelength division multiplexed(WDM) and dense wavelength division multiplexed (DWDM) opticaltransmission systems relies on the availability of optical transmittersoperating at precisely controlled wavelengths. Such transmitterstypically use wavelength selected laser diodes as the optical source.Typical DWDM systems operate with many wavelengths, uniformly spaced byfrequency, operating in the so-called C-band and/or S-band or L-band,windows of gain provided by the erbium-doped fiber amplifier. Forexample, in accordance with the optical communications standards set bythe International Telecommunications Union (ITU), a DWDM system mayoperate with 80 channels of different wavelengths uniformly spaced by achannel spacing of 50 GHz. It is anticipated that future systems willoperate with greater numbers of channels and with smaller interchannelspacings.

[0003] It is also desirable that DWDM systems operate with lasers thatare locked to the particular channel frequency, without long-term drift.If the wavelength of the laser drifts, the system may sufferunacceptable crosstalk in adjacent channels. A typical requirement isthat the frequency of the laser output does not drift by more than 3 GHzover a span of twenty years. A laser diode will naturally drift by anamount considerably greater than 3 GHz over this time period, the actualamount of the drift being dependent on specific aging characteristics ofthe laser.

[0004] This time-dependent frequency drift can be minimized, if notavoided altogether, by actively controlling the laser wavelength. Activecontrol may include deliberately changing an operating characteristic ofthe device that affects the output wavelength, such as temperature orcurrent, to compensate for the natural frequency drift. This requires afixed, known frequency reference for comparison of the emissionwavelength from the laser. It is often desirable for network managementpurposes that each laser be locked locally to its own reference,preferably within the laser diode package. It is also desirable in somecircumstances that a single, standard reference assembly can be usedwith any one of a multitude of fixed frequencies, or with a tunablelaser capable of operation at any such wavelength. This enables a widelytunable laser to be used at any of the channel frequencies, and avoidsthe requirement that the laser be selected to operate within only asmall fraction of the channels.

[0005] Various wavelength locking solutions have been proposed,including the use of crystal gratings and fiber Bragg gratings,interference filters and etalons. Crystal and fiber Bragg gratings areoptimized for operation at one wavelength and do not fit easily into astandard laser diode package. Interference filters can fit inside alaser package, but are typically also optimized for only one wavelength.

[0006] Fabry-Perot etalons have been the subject of significantdevelopment in wavelength locking schemes. These devices demonstrate atransmission curve that has periodical maxima when plotted against lightfrequency. This periodical transmission curve needs to be tuned to matchthe required ITU-grid frequency spacing, which is done either by tiltingthe etalon or changing its temperature. However, tuning the etalon is asensitive and complicated process which requires active alignment orprecise temperature control. In addition the tuning process becomes moresensitive as the number of ITU channels increases or the interchannelspacing decreases.

[0007] Therefore, there is a need for an approach to stabilizing thewavelength of a laser output that is low cost, easily adjustable inproduction and is sufficiently compact to fit into a standard laserpackage. Furthermore, since the wavelength locker may be used tostabilize the output from a backup laser diode that substitutes for alaser that has failed, the wavelength locker should be able to operateat any wavelength over the DWDM band.

SUMMARY OF THE INVENTION

[0008] Generally, the present invention relates to an apparatus andmethod for locking the wavelength of a laser that uses afringe-producing optical element in the output beam of the laser. Thefringe producing optical element produces a fringe pattern in a secondlight beam derived from the output beam. The fringe pattern is detectedusing a detector unit. Signals generated by the detector unit are usedto generate a laser tuning control signal that tunes the laser to adesired operating wavelength.

[0009] One embodiment of the invention is directed to a laser systemthat includes a laser producing a beam of output light and a detectorunit. A fringe producing optical element is disposed in the beam ofoutput light to direct a portion of the beam of output light to thedetector unit as a second light beam. The fringe-producing opticalelement causes an interference pattern in the second light beam.

[0010] Another embodiment of the invention is directed to a opticalcommunications system that includes an optical communicationstransmitter unit having one or more laser units, an opticalcommunications receiver unit, and an optical fiber communications linkcoupled to transfer optical communications signals from the opticalcommunications transmitter unit to the optical communications receiverunit. At least one of the one or more laser units produces a laseroutput beam and has a wavelength stabilizing unit. The wavelengthstabilizing unit includes a detector unit and a fringe -producingoptical element disposed in the laser output beam to direct a portion ofthe laser output beam to the detector unit as a second light beam. Thefringe-producing optical element causes an interference pattern in thesecond light beam. A control unit is coupled to receive detectionsignals from the detector unit and generates a laser frequency controlsignal for controlling wavelength of the at least one of the one or morelaser units,

[0011] Another embodiment of the invention is directed to a method ofstabilizing an operating frequency of an output light beam produced by alaser. The method includes deflecting a portion of the output light beamas a second light beam using a fringe-producing optical element, thefringe-producing optical element causing an interference fringe patternin the second light beam. Portions of the interference fringe patternare detected using a detector unit and detector signals are produced inresponse to the detected portions of the interference fringe pattern. Afrequency control signal is generated in response to the detectorsignals and the laser is tuned in response to the frequency controlsignal so that the operating frequency of the output light beam issubstantially at a desired value.

[0012] Another embodiment of the invention is directed to a method ofstabilizing an operating frequency of an output light beam produced by alaser. The method includes deflecting a portion of the output light beamas a second light beam using a fringe-producing optical element. Thefringe-producing optical element causes an interference fringe patternin the second light beam. The operating frequency of the output lightbeam is stabilized using the interference pattern.

[0013] Another embodiment of the invention is directed to a system forstabilizing an operating frequency of an output light beam produced by alaser. The system includes deflecting means for deflecting a portion ofthe output light beam as a second light beam and for producing aninterference fringe pattern in the second light beam. The system alsoincludes means for stabilizing the operating frequency of the outputlight beam using the interference fringe pattern.

[0014] The above summary of the present invention is not intended todescribe each illustrated embodiment or every implementation of thepresent invention. The figures and the detailed description which followmore particularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The invention may be more completely understood in considerationof the following detailed description of various embodiments of theinvention in connection with the accompanying drawings, in which:

[0016]FIG. 1 schematically illustrates an optical communications systemthat includes a laser whose wavelength is stabilized according to thepresent invention;

[0017]FIG. 2 is a block schematic diagram illustrating elements of afrequency stabilized laser;

[0018]FIG. 3 schematically illustrates one approach to generating anoptical signal used for wavelength locking, according to the presentinvention;

[0019]FIG. 4 schematically illustrates the spatial pattern of lightgenerated by a non-planar etalon and an embodiment of a detector unitused for detecting the spatial pattern of light according to the presentinvention;

[0020]FIG. 5 schematically illustrates the spatial patterns of lightgenerated by a non-planar etalon at different wavelengths and anotherembodiment of a detector unit used for detecting the spatial pattern oflight according to the present invention;

[0021]FIG. 6 shows a graph illustrating compound detector signals asfunction of frequency;

[0022]FIG. 7 shows a graph illustrating phase signals obtained from thecompound signals shown in FIG. 6;

[0023]FIG. 8 shows a graph illustrating feedback signals used forstabilizing frequency of the laser;

[0024]FIGS. 9 and 10 schematically illustrate other approaches togenerating an optical signal used for wavelength locking using areflective wedge mirror according to the present invention;

[0025]FIG. 11 schematically illustrates a diffractive NPE according toan embodiment of the invention;

[0026]FIG. 12 schematically illustrates a Fresnel NPE according to anembodiment of the invention; and

[0027]FIG. 13 schematically illustrates a binary NPE according to anembodiment of the invention.

[0028] While the invention is amenable to various modifications andalternative forms, specifics thereof have been shown by way of examplein the drawings and will be described in detail. It should beunderstood, however, that the intention is not to limit the invention tothe particular embodiments described. On the contrary, the intention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

[0029] The invention is related to another invention entitled “METHODFOR CONTINUOUS WAVELENGTH LOCKING”, filed on even date herewith by G.Hedin, having a U.S. patent application Ser. No. ______, and “ROBUSTWAVELENGTH LOCKER FOR CONTROL OF LASER WAVELENGTH” filed on even dateherewith by G. Hedin and J. Tegin, having a U.S. patent application Ser.No. ______, both of which are incorporated herein by reference.

[0030] The invention provides a compact wavelength monitoring assemblyfor use in conjunction with tunable laser sources. The wavelengthmonitoring assembly is used for stabilization of the emission wavelengthand for locking the wavelength to an electrical reference signal. Thecompact optical configuration of the assembly makes the deviceparticularly well suited for incorporation inside standard packages thatare used in telecommunications applications.

[0031] According to one embodiment of the invention, a fringe-producingoptical element is illuminated and produces an interference fringepattern. The fringe-producing optical element may be an etalon,typically a solid etalon, that has one surface non-parallel with respectto the other surface. The non-parallel surface may be curved, stepped orflat. As the laser frequency, f, changes, the interference fringepattern moves and is monitored by a set of detectors having a spatialdistribution chosen to match the interference pattern, thereby samplingthe interference pattern at fixed positions with known spatial phasedifferences. From the detector signals, it is possible to determine thespatial phase φ (fringe position) of the interference pattern. The phasesignal changes by 2π for a laser frequency shift equal to one freespectral range (FSR). At least three detector signals are used touniquely identify the phase of an interference pattern.

[0032] This approach offers advantages over prior approaches towavelength locking. For example, the output power from the laser may beinferred from the detected signals, and so there is no need for aseparate power monitor. Furthermore, the frequency locking system basedon the use of at least three signals is robust, and permits locking toany desired frequency. Since the wavelength and the frequency of lightare related, it will be appreciated that these terms, in some instances,may be used interchangeably.

[0033] Another advantage is that no etalon tuning is needed since theabsolute value of FSR only affects the derivative dφ/df or,equivalently, dφ/dλ. In other words, absolute value of FSR affects thespeed of the fringe pattern motion with changing frequency. Passivealignment of the etalon is possible due to the weak dependence on FSR.Another advantage is that the locker is able to lock frequencies on anirregular frequency grid, since no matching of the etalon transmissioncurve to the frequency grid is needed.

[0034] A laser stabilized using the present invention may be employed ina DWDM communications system 100, schematically illustrated in FIG. 1.The system 100 includes a WDM transmitter unit 102 that includes anumber of lasers 104 a-104 n operating at different wavelengths, λ1-λn.Any of the lasers 104 a-104 n may be a laser whose wavelength isstabilized according the present invention. In addition, one or morespare lasers 105 may operate as a substitute if any of the lasers 104a-104 n fail. The lasers 104 a-104 n and 105 may each include modulatorsfor modulating information onto the respective output light beams. Theoutputs from the lasers 104 a-104 n, 105 may be combined in a DWDMcombiner arrangement 106 and launched as a DWDM signal into an opticalfiber communications link 108 that is coupled to a DWDM receiver 110.The fiber link 108 may include one or more fiber amplifier stages 112 toamplify the DWDM signal as it propagates to the DWDM receiver 110. Otherelements, such as isolators, switches, add/drop multiplexers and thelike may also be disposed along the fiber link 108. The DWDM receiver110 demultiplexes the received DWDM signal in a demultiplexer 114 anddirects signals at different wavelengths λ1-λn to respective channeldetectors 116 a-116 n.

[0035] A block schematic diagram showing various elements of a frequencystabilized laser unit 200 is illustrated in FIG. 2. A laser 202generates an output light beam 204 that is directed to a wavelengthdetector unit 206, which generates an output signal 208 determined bythe wavelength of the light in the light beam 204.

[0036] The laser may be any suitable type of semiconductor laser thatproduces a tunable output. Monolithically tunable lasers are often usedin optical communications applications, such as distributed Braggreflector (DBR) lasers, grating coupled, sampled Bragg reflector (GCSR)lasers, for example as described in “74 nm Wavelength Tuning Range of anInGaAsP Vertical Grating Assisted Codirectional Coupler Laser with RearSampled Grating Reflector” by M. Oberg et al., IEEE Photonics TechnologyLetters, Vol. 5, No. 7, pp. 735-738, July 1993, incorporated herein byreference, and in U.S. Pat. No. 5,621,828, also incorporated herein byreference, and vernier, dual DBR lasers, for example as described inU.S. Pat. No. 4,896,325.

[0037] A residual output beam 210, passing from the wavelength detectorunit 206, may carry optical output power not used in the determinationof the wavelength. The residual output beam 210 may be used as theuseful optical output from the laser 202. Where the output light beam204 carries the main optical output from the laser 202, the wavelengthdetector unit 206 advantageously uses only a small fraction, for examplea few percent, of the output light beam 204, in order to increase thepower in the residual output beam 210.

[0038] A wavelength analyzer unit 212 receives and analyzes the outputsignal 208 from the wavelength detector unit 206 to determine thewavelength of the light beam 204. The analyzer 212 typically generatesan error signal 214 that is directed to a wavelength controller. Thesize of the error signal typically indicates the amount by which themeasured wavelength of the laser deviates from a desired value. Theerror signal 214 is directed to a tuning controller 216 that isconnected to the laser 202 and controls the operating wavelength of thelaser 202.

[0039] The wavelength tuning controller 216 may be incorporated with alaser controller 218 that includes the power supply 220 for providingpower to the laser 202 and a temperature controller 222 that controlsthe temperature of the laser 202. The laser 202 may be coupled, forexample, to a thermoelectric device 224 or other type of device foradjusting temperature.

[0040] The laser 202 and wavelength detector unit 206 may be enclosedwithin a housing 226 to prevent environmental effects from affecting theoperation of the laser 202 and the wavelength detector unit 206. Thedevice 224 for adjusting operating temperature may also be locatedwithin the housing 226.

[0041] One particular embodiment of a wavelength stabilized detectorunit is illustrated in FIG. 3. The laser 302 generates an output lightbeam 304 whose divergence is reduced by a focusing unit 306. Thefocusing unit may include one or more lenses. The light beam 308 passingout of the focusing unit 306 may be approximately collimated, or may beconvergent or divergent. For purposes of clarity, it is assumed in thefollowing description that the light beam 308 is collimated. It will beappreciated, however, that the present invention also operatesconvergent and divergent light.

[0042] The light beam 308 may pass through an optical isolator 310,which permits light to pass in the forward direction, but which preventslight passing in the backwards direction towards the laser 302. Thisprevents reflected light from re-entering the cavity of the laser 302and adversely affecting the stability of the light 304 output from thelaser. After the light 308 has passed through the isolator 310, abeamsplitter 312 splits a fraction 314 of the light 308 as a probe beam.The beamsplitter 312 may be, for example, a flat piece of glass with oneside antireflection coated, where the probe beam 314 is split off byreflection from the uncoated surface. The beamsplitter 312 may also be abeamsplitter cube or any other suitable form of optical element thatsamples the light 308 from the laser 302.

[0043] The light transmitted through the beamsplitter 312 forms theresidual beam 318, which may be directed to a focusing unit 320,typically one or more lenses, for coupling into the output fiber 322.Typically, the optical power coupled into the output fiber 322constitutes the useful output light from the laser 302 and may be usedto form an optical communications signal. The output fiber 322 may leadfirst to a modulator for imposing information on the light propagatingalong the fiber 322. Typically, the optical power in the probe beam 314is around a few percent of the power in the residual beam 318.

[0044] The probe beam 314 is directed to a fringe-producing opticalelement 316, such as a non-parallel etalon (NPE). Some types offringe-producing optical elements are discussed further in U.S. patentapplication Ser. No. 09/871,230, incorporated herein by reference. Inthe illustrated embodiment, the fringe-producing optical element 316 isa NPE. When illuminated with a beam of light, a fringe-producing opticalelement produces second beam of light that includes an interferencepattern, having interference fringes. The second beam of light may bereflected from the fringe-producing optical element or may betransmitted from the fringe-producing element. Typically, the secondbeam is formed by two interfering beam components arising from twodifferent surfaces of the fringe-producing element.

[0045] The probe beam 314 propagates to the NPE 316, which operates as aspatial wavelength selective filter. The NPE 316 is formed from materialthat transmits light at the output wavelength of the laser 302, forexample glass or plastic. An NPE has surfaces that contain portions thatare non-parallel, and may be wedged or may include at least onenon-planar surface. A non-planar surface may assume any type of shape,including spherical, aspherical, toroidal, cylindrical, or steppedshapes. If the etalon includes a non-planar surface, it may be referredto as a non-planar etalon. A NPE having a stepped surface may be, forexample, a binary optic etalon or a Fresnel etalon, as described below.A NPE having a stepped surface may also have a wedged or curved profile.

[0046] In the illustrated embodiment, the NPE 316 has first and secondfaces 324 and 326 that are flat, but not parallel to each other, and sothe NPE 316 is wedged. The reflectivity of the first face 324 is R1 andthe reflectivity of the second face 326 is R2. The magnitudes of thesurface reflectivities, R1 and R2, may be equal, although they need notbe equal. For example, if the NPE 316 is formed from glass havinguncoated surfaces 324 and 326, then the reflectivities R1 and R2 aredetermined by the difference in refractive index between the material ofthe NPE 316 and the medium in which the NPE 316 is immersed. If the NPE316 is formed from glass, having a refractive index of around 1.5, andis immersed in air, then the reflectivity of each surface 324 and 326 isaround 4%, when the angle of incidence on the faces 324 and 326 is closeto normal. It will be appreciated that the surfaces 324 and 326 may alsobe provided with coatings having specific reflective values in the rangefrom greater than 0% to almost 100%. In the present invention, thereflectivities R1 and R2 may lie in the range 1%-50%, and morepreferably in the range 10%-25%. The values of R1 and R2, however, neednot be restricted to these ranges.

[0047] The probe beam 314 is partially reflected at the first surface324 and partially transmitted into the material of the NPE 316. Thispartially transmitted beam propagates towards the second surface 326where it is again partially transmitted and partially reflected. Thelight undergoes a series of internal reflections within the NPE 112. Thetotal optical power reflected from the NPE 316 towards the beamsplitter312 may be determined from coherent addition of all partially reflectedbeams. Where the reflectivity is low, however, for example 10% or lower,then the light reflected by the NPE 316 is primarily the light that wasreflected only once, by either the first or second surface 324 or 326.In the illustrated embodiment, the light reflected by the first surface324 is labeled beam 328 (solid lines) and the light reflected by thesecond surface 326 is labeled as 330 (dotted lines), although it isunderstood that a component of the reflected signal corresponds to lightthat was reflected within the NPE 316 multiple times.

[0048] The beams 328 and 330 pass through the beamsplitter 312 and areincident on a detector unit 332 that includes at least three detectorelements 334, also referred to as detector pixels. The pixels 334 may bearranged in an array. Furthermore, the shape of the pixels 334 may beadapted so as to increase the overlap with the interference fringes ofthe interference pattern formed in the light by the NPE 316. Forexample, where the NPE 316 is wedged with flat surfaces, theinterference pattern includes parallel fringes and the pixels 334 may berectangular, and elongated in the direction perpendicular to the fringeseparation. Where the NPE 316 has a curved surface, the resultinginterference fringes may be curved and the pixels may be curved to matchthe curves of the interference fringes.

[0049] The detector unit 332 may be mounted on a detector carrier 336,which provides mechanical support for the detector unit 332 which mayalso provide electrical contact between the detector unit 332 and thecontrol unit (not shown). The carrier 336 may be formed from anelectrically insulating material, such as alumina or the like, and maybe provided with bond pads for forming electrical contacts.

[0050] The detector carrier 336, beamsplitter 312 and NPE 316 may all bemounted on a mounting plate 338 that provides a thermal, electricaland/or mechanical interface between the NPE 316, detector carrier 332and beamsplitter 312, and the rest of the laser housing 226. The designof the mounting plate 338 may provide solder and bond pads and may alsoinclude electrical circuit lines for electrical connections. Themounting plate 338 may be formed from an electrically insulatingmaterial, although it is also advantageous that the carrier be a goodthermal conductor. Accordingly, the mounting plate may be formed fromalumina, aluminum nitride, or some other ceramic having good thermalconductive properties.

[0051] Interference between light reflected from the first and secondsurfaces 324 and 326 results in spatial modulation of the light incidenton the detector unit 332. The spatial modulation is typically periodic,although depending on the curvature of the surfaces 324 and 326 of theNPE 316, the period may vary across the detector unit 332. In theillustrated example of a wedged reflector, having flat surfaces 324 and326, the period of the interference pattern is constant across thedetector unit 332.

[0052] The detector unit 332 has at least three pixels 334 that detectdifferent parts of the spatially modulated interference pattern. Thethree, or more, pixels 334 are positioned so as to detect differentparts of the interference pattern that correspond to different spatialphase. The NPE 316 and detector unit 332 are advantageously designed tomatch each other so that the spacing of the pixels 334 is such that thepixels 334 are positioned to detect evenly spaced portions of a periodof the interference pattern. For example, if the detector unit 332 usesthree pixels, then uniform spacing between pixels, which permitssimultaneous power monitoring, corresponds to a phase difference of theperiodic interference pattern of about 2π/3. More generally, where thedetector unit 332 employs n pixels 334, then the spacing between pixels334 corresponds to 2π/n. It will be appreciated that adjacent pixels 334may be have different spacings, for example may also be spaced apart bya distance corresponding to mπ+2kπ/n, where k and m are integers.

[0053] The diffraction and pointing sensitivity is much reduced sinceonly linear spatial modulation in one direction is measured. Thedegradation properties are improved since all pixels 334 aremanufactured on the same chip. The manufacturing and assembling of thewavelocker system requires only alignment of the etalon by rotationabout one axis, and alignment of the detectors by translating alonganother axis, in order to correctly map the pattern on to the detectors.

[0054] In an example illustrated in FIG. 4, an interference pattern 402is shown as light intensity (in arbitrary units) as a function ofspatial position, measured in mm. The period of the interference pattern402 is P and, in this example where three pixels 434 a, 434 b and 434 care used, the spacing between the pixels 434 a, 434 b and 434 c is P/3.The interference pattern 402 was obtained from simulations of a NPEformed from BK7 glass, having a refractive index of 1.51 at a designwavelength of 1.55 μm. The assumed thickness of the NPE was 2 mm and thewedge angle was 0.2°, resulting in a fringe spacing in the interferencepattern of about 150 μm. The surface reflectivity was in the range10%-20%. The spacing between pixels 334 of the detector unit 332 is 50μm. The pixels may be 500 μm high and 25 μm wide.

[0055] The choice of reflection coefficient of the NPE is a compromisebetween fringe shape, modulation depth, optical power and ghost fringes.Sinusoidal patterns are often preferred for various feedback detectionschemes, and are obtained for reflection coefficients less than about10%. Near-sinusodial fringe patterns are obtained for reflectioncoefficients in the range of about 10%-25%, and when the reflectioncoefficient is greater than about 25%, the fringe pattern assumes aperiodical Lorentzian shape, characterized by sharp peaks and broadvalleys.

[0056] The modulation depth MD, is given by the expressionMD=(Imax−Imin)/(Imax+Imin). The MD of a reflected interference patternis close to 100% where the reflection coefficient is less than about25%. On the other hand, the modulation depth of an interference patterntransmitted through the NPE increases, at least for small values of R,as 2×R. Therefore, an uncoated NPE having a surface reflection of 4%manifests a MD of 8%. Thus, the fraction of optical power in thereflected pattern is about 2×R, and about 1−(2×R) in the transmittedpattern. Therefore, the ratio of transmitted to reflected power is about11 for an uncoated NPE having a surface reflection of about 4%.

[0057] Ghost fringes may occur for higher values of reflection,typically about 25% and more, due to multiple reflections in the NPE.These ghost fringes at best create a background that reduces the MD, andat worst cause higher spatial frequencies in the interference pattern.Therefore, when detecting a reflected interference pattern, the surfacereflectivity of the NPE is advantageously low to increase the MD and tomake the fringe pattern more closely sinusoidal. It will be understoodthat the surface reflectivity may be have a lower boundary set by theminimum acceptable level of optical power at the detector chip. For aconfiguration such as that as illustrated in FIG. 3, an optimal valuefor the surface reflectivity may lie in the range 5%-15%. Where thefringe pattern transmitted through the NPE is detected, the value of Rshould be high to increase the MD, but not so high as to cause higherorder distortions of the fringe pattern. A typical value of reflectivityfor an NPE operating in transmission is around 30%.

[0058] The detector unit 332 may be positioned behind the NPE 316 todetect the interference pattern on the light transmitted through the NPE316. As a result of using reflectivities less than about 25% on thesurfaces 324 and 326, however, the interference pattern of lighttransmitted through the NPE 316 has a relatively low modulation depth.In comparison, the modulation depth of the interference patternreflected from the NPE 316 is relatively high. Therefore, use of theinterference pattern reflected from the NPE 316 provides advantages insignal to noise.

[0059] The frequency, f, of the light output from the laser 302 may bepresented in terms of the free spectral range (FSR) of the NPE 316 asf=k×FSR+f′, where k is an integer value. The value of the FSR may beobtained from the expression FSR=c/(2n₀I), where n₀ is the refractiveindex of the NPE 316 and I is the thickness of the NPE 316. Therefore,the phase of the interference pattern 402 relative to the pixels 434 a,434 b and 434 c is proportional to f′. The phase of the interferencepattern 402 is, therefore, a direct measurement of the output laserfrequency, at least over a frequency range equal to the FSR.

[0060] The output signal from the first pixel 434 a may be termed R, theoutput signal from the second pixel 434 b may be termed S and the outputfrom the third pixel 434 c may be termed T. Where the number of pixels434 is three or more, the sum of the optical signals on the pixels 434is a direct measurement of the average irradiance on to the detectorchip. Consequently, so long as there is adequate calibration, forexample to extract variations in signal level due to the intensityenvelope 404, the sum of the signals from the pixels (R+S+T) isproportional to the total laser power.

[0061] The effect of changing the wavelength is described with referenceto FIG. 5, which shows the interference pattern 502 formed when thelaser output is at a first frequency, and a second interference pattern504 formed when the laser output is changed to a second frequency. Thefringe pattern is seen to move across the detector unit 332 when thelaser frequency changes. This movement is detected by the detector unit332, since the signals produced by the different pixels 434 change.

[0062] Furthermore, in this embodiment, the pixels 534 a-534 f cover aspan of two periods of the interference patterns 402 and 502. Theoutputs from two pixels spaced apart by the period may be combined. Forexample, the outputs from pixels 534 a and 534 d may be combined to formsignal R, the outputs from pixels 534 b and 534 e may be combined toform signal S and the outputs from pixels 534 c and 534 f may becombined to form signal T. The inter-pixel spacing for the pixels 534a-534 f is P/3. An advantage to using more than one pixel to detect aparticular phase portion of the interference pattern 502 and 504 is thatthe signal to noise ratio may be increased.

[0063] Another illustration of the effect of changing the wavelength ofthe light being detected by the detector unit 332 is presented in FIG.6, which shows the values of R, S, and T as functions of the frequencyof the light incident on the NPE 316. The values of R (curve 602), S(curve 604), and T (curve 606) all vary periodically with increasingfrequency of the incident light. Furthermore, the signals R, S, and Tare equally spaced from each other.

[0064] One approach to describing the signals R, S, and T is:

[0065] R=I₀(1+cos(φ+α))

[0066] S=I₀(1+cos(φ)), and

[0067] T=I₀(1+cos(φ−α))

[0068] where φ is the phase of the interference fringe, α is the thephase difference between adjacent pixels and I₀ is the average lightirradiance. For three-pixel detection, where the detectors are spacedevenly over a period of the interference pattern, a has a value equal to2π/3 (120°). The value of φ depends on the free spectral range (FSR) ofthe etalon and the frequency, f, of the incoming light through theexpression: φ=4πf mod (FSR), where f mod (FSR) is the remainder afterhighest possible intergral number of FSRs is substracted from thefrequency. For example, if the frequency is given by f=191,045 GHz, andthe FSR is 100 GHz, then f mod (FSR) is 45 GHz.

[0069] The following signals may be calculated from the measured valuesof R, S, and T. First, the value I is given by:

I=(R+S+T)/3  (1)

[0070] where I is independent of φ and is equal to I₀. Therefore, I isproportional to the laser power incident on the detector unit 332 and,consequently, may be proportional to the output power from the laser.The average signal 608 in FIG. 6 represents I.

[0071] Another useful signal is cos(φ) where:

cos(φ)=(S−I)/I  (2)

[0072] and another useful signal is:

sin(φ)=(T−R)/(I{square root}3).  (3)

[0073] Therefore, one value of φ may be calculated as:

φ=tan⁻¹[sin(φ)/cos(φ)]  (4)

[0074] Examples of curves showing the relative values of the phasesignals φ (curve 702), sin(φ) (curve 704) and cos(φ) (curve 706) fordifferent frequencies of incident laser light are presented in FIG. 7,for the same example of NPE discussed previously.

[0075] To lock the laser light to a certain frequency, the laser isfirst tuned to the locking value, f₀, in other words that value to whichit is desired to lock the laser, and the detector signals, R, S, and T,for that frequency are stored as R₀, S₀ and T₀.

[0076] These values of R₀, S₀ and T₀ are then used in expressions (2),(3) and (4) to calculate reference phase signals, sin(φ₀), cos(φ₀) andφ₀. The reference phase signals may be used to calculate a feed-backsignal along with the phase signals, sin(φ), cos(φ) and φ, from themeasured signals.

[0077] Transformed signals may be calculated by forming a transformedphase, φ′=φ−φ₀. Therefore, transform equations are as follows:

cos(φ′)=cos(φ₀)cos(φ)+sin (φ₀)sin(φ)  (6)

sin(φ′)=−sin(φ₀)cos(φ)+cos(φ₀)sin(φ)  (7)

φ′=tan⁻¹ (sin(φ′)/cos(φ′))  (8)

[0078] Two types of feed-back signals, used as error signals 214, may beformed from these expressions. The first feedback signal is sin (φ′),expression (7). This has a capture range of ±FSR/2, and has a nonlinearresponse. This is termed sine feedback. The other feedback signal is φ′,as provided in expression (8). This feedback signal has a capture rangeof ±FSR/4 and has a linear response. This is termed phase feedback.Phase feedback provides the advantage over sine feedback that theresponse is linear, however, more processing is required to calculate φ′than is required to calculate sin(φ′). The values of sin (φ′), curve802, and φ′, curve 804, are shown in FIG. 8, plotted against frequencyof the light being locked. The signal cos(φ′) may also be used as afeedback signal.

[0079] It will be appreciated that a similar analysis may be performedusing four or more detector elements spaced to sample portions of theinterference pattern corresponding to different values of phase over aperiod. Such analysis yields feedback equations corresponding toexpressions (6)-(8).

[0080] One of the advantages with this approach to locking the frequencyof the laser include is that any frequency may be locked on to with thesame capture range and response slope, and that the absolute thicknessand tilt of the etalon is a weak variable that only determines the slopeof the feed-back signal at the locking point and the absolute capturerange. Another advantage is that the intensity may be inferred from thesignals received by the detector unit 332 that is used to measure thewavelength, and no additional power monitor is required.

[0081] Where the NPE has at least one curved surface, the spacingbetween the maxima of the interference pattern may not be constant. Insuch a case, the spacings between adjacent pixels in the detector unitneed not be constant, but may be selected to suit the nonlinearity ofthe interference pattern. For interference patterns where thenonlinearity is relatively small, the pixels may still be spaced apartby a uniform inter-pixel spacing: the feedback scheme is robust and doesnot require exact inter-pixel spacing for operation. For example, in athree pixel detector scheme, adequate feedback may still be providedwhere the spacing between pixels is 2π/3±π/6, even where the NPE is awedged etalon. The feedback technique may also operate outside thisrange, but with decreased effectiveness.

[0082] Another embodiment of a wavelength detector unit 900 isschematically illustrated in FIG. 9. In this embodiment, light 904diverges from a laser 902 and is substantially collimated by a focusingunit 906. The collimated beam 908 passes through a NPE 912 that reflectslight from both surfaces 911 and 913. In the illustrated embodiment, theNPE 912 is a reflective wedge, although it may also include one or morecurved surfaces. The light 918 that is not reflected by the NPE 912 maybe focused by a lens unit 920 to an output fiber 922.

[0083] The light 928 (solid lines) reflected from the first surface 911and the light 930 (dashed lines) reflected from the second surface 913may be reflected by a reflector 916 to the detector unit 932. Thedetector unit 932 has three or more pixels 934 to detect theinterference pattern caused by the interference between the reflectedlight beams 928 and 930.

[0084] In one particular embodiment, the NPE 912 may be formed from BK7glass having a refractive index of 1.51 at the design wavelength of 1.55μm. A wedge angle of 0.2° produces a fringe spacing in the resultantinterference pattern of about 150 μm. A thickness of 1 mm gives an FSRof 100 GHz, while a thickness of 2 mm gives an FSR of 50 GHz. Thereflectivity of the surfaces 911 and 913 may be in the range ofapproximately 1%-2% in order to reduce insertion loss in the beam 918.The reflectivity may be less than 1%, so long as the minimum powerrequirements of the detector unit 932 are satisfied. Higher values ofreflectivity may not provide significant benefit in terms of signal tonoise or reduction of higher order reflections, but do increase theinsertion loss.

[0085] The angle of incidence on the NPE 912 may be around 15°, althoughany suitable angle may be used, depending on the beam diameter and thethickness of the NPE 912. Where the beam diameter is fixed, a thickerNPE 912 is advantageously tilted at a smaller angle, to ensure goodoverlap between the beams reflected from the two surfaces 911 and 913.Since the reflectivity of the surfaces 911 and 913 is typically low, anyinterference or etalon effects in the beam 918 transmitted to the outputfiber 922 resulting from the NPE 912 may be regarded as beinginsignificant.

[0086] One of the advantages of placing the NPE 912 directly in the beam908 is that all light reflected out of the beam 908 by the NPE 912 isincident on the detector unit 932, and therefore the use of the tappedlight is very efficient. This contrasts, for example, with theembodiment illustrated in FIG. 3, where light transmitted through theNPE 316 is not used for measuring the wavelength of the light.Furthermore, passing the light 328 and 330 through the beamsplitter 312results in additional losses.

[0087] Another embodiment of wavelength detector unit 1000 isillustrated in FIG. 10. Here, the folding mirror 916 has been omittedand the light 928 and 930 reflected by the NPE 912 is incident directlyon the detector unit 932. The detector unit 932 is positionedsufficiently far from the NPE 912 that the detector unit 932 receivesthe reflected light 928 and 930 but does not occlude any of the light908 incident on the NPE 912.

[0088] The interference pattern from the two reflected beams 928 and 930occurs only where the two beams 928 and 930 overlap. Reflection in areflective wedge results in a displacement, δ, between the beamreflected from the first surface and the beam reflected from the secondsurface, given by δ=2tα/n₀, where t is the etalon thickness, a is theincident angle, and n₀ is the refractive index of the reflective wedge.In the expression for δ, it has been assumed that the wedge angle of theNPE 912 is sufficiently small that the contribution to δ from the wedgedshape of the wedge may be neglected.

[0089] For a 1 mm thick glass etalon at an angle of incident of 15°, thedisplacement is about 330 μm, and so the interference pattern has awidth of about 670 μm. Such an etalon has a FSR of 100 MHz. To increasethe resolution of the wavelength locker, in other words decreasing theFSR, the incident angle a is reduced to allow for a thicker etalon whilemaintaining the same width of the interference pattern. Another approachis to use an optically denser etalon material. Reducing a, however,increases the length of the wavelength locker since the detector unit932 must be placed further away from the etalon in order not to shadethe incoming light 908. By using a folding mirror 916, the overalllength requirement can be some what reduced.

[0090] In general, the wedge angle of the NPE 912 is small, in manycases less than 1°, and so the walk-off between the two beams 928 and930 propagating from the NPE 912 to the detector unit 932 is very small,if not negligible. Therefore, although they have been described asseparate beams, the reflected beams 928 and 930 may together be regardedas a single beam, derived from the output beam 980 of the laser, thatcontains an interference pattern.

[0091] Another embodiment of reflector that may be used in thewavelength locker is schematically illustrated in FIG. 11. The reflectoris a diffractive etalon 1100 having a first side 1102 and a second side1104. The first and second sides 1102 and 1104 may or may not beparallel to each other. The light 1106 from the laser to be wavelengthlocked is incident on the first side 1102. A diffracting structure 1108,having a grating period d₁, is disposed on the first side 1102, so thata portion of the incident light 1106 is diffracted as beam 1110 (solidlines), at an angle a, to the incident light 1106.

[0092] The light 1112 that is transmitted through the first side 1102 isincident on the second side 1104. The second side 1104 is provided witha second diffracting structure 1114, having a grating period d₂, so thatsome of the light incident on the second side 1104 is diffracted as beam1116 (dashed lines) at an angle α₂ relative to the incident light 1106.

[0093] The two beams 1110 and 1116 overlap and interfere to cause afringe pattern that may be detected by a multi-element detector unit1118 to produce detection signals that are used for determining thewavelength of the light 1106.

[0094] The diffractive etalon 1100 produces two beams 1110 and 1116 thatpropagate in different directions and, therefore, may be considered tobe a wedged reflector, even though the two surfaces 1102 and 1104 may beparallel. It will be appreciated that the light 1106 incident on thediffractive etalon 1100 need not be incident at normal incidence.

[0095] The diffracting structures 1108 and 1114 may reflectivelydiffract, as illustrated in FIG. 11, in which case the diffractiveetalon 1100 may be used in the embodiments of wavelength locker similarto those illustrated in FIGS. 3, 9 and 10. The diffracting structures1108 and 1114 may also diffract in transmission, rather than reflection,in which case the diffractive etalon 1100 may be employed in otherconfigurations.

[0096] Two other types of fringe-producing optical elements that may beused in the present invention are illustrated in FIGS. 12 and 13. AFresnel etalon 1200, illustrated in FIG. 12, is a NPE that may be usedin a wavelength locker. The Fresnel etalon 1200 has first and secondsurfaces 1202 and 1204. One of the surfaces, the second surface 1204 inthe illustrated example, includes a ridged pattern of long surfaces 1206and short surfaces 1208. The long surfaces 1206 are not parallel to thefirst surface 1202, but are at an angle relative to the first surface1202. The long surfaces 1206 may be flat or curved. The averagethickness across the Fresnel etalon 1200 may be constant, or may vary.It will be appreciated that one or both of the surfaces 1202 and 1204may be provided with a ridged pattern of long and short surfaces.

[0097] In use, light 1210 is incident on the Fresnel etalon 1200. Thefirst surface 1202 reflects a portion of the light 1210 as beam 1212(solid lines) and the second surface reflects a portion of the light1210 as beam 1214 (dashed lines). The light that is not reflected byeither the first or second surfaces 1202 and 1204 is transmitted as beam1216. The two reflected beams 1212 and 1214 overlap and interfere,causing an interference pattern that may be detected by a detector unit.

[0098] A binary etalon 1300, illustrated in FIG. 13, is a NPE that maybe used in a wavelength locker. The binary etalon 1300 has first andsecond surfaces 1302 and 1304. One of the surfaces, the second surface1304 in the illustrated example, includes a stepped pattern of longsurfaces 1306 and short surfaces 1308. The long surfaces 1306 areparallel to the first surface 1302, while the short surfaces 1308 arenot parallel to the first surface 1302. The average thickness across thebinary etalon 1300 varies from one side of the etalon 1300 to the other.It will be appreciated that one or both of the surfaces 1302 and 1304may be provided with a stepped pattern of long and short surfaces.

[0099] In use, light 1310 is incident on the binary etalon 1300. Thefirst surface 1302 reflects a portion of the light 1310 as beam 1312(solid lines) and the second surface reflects a portion of the light1310 as beam 1314 (dashed lines). The light that is not reflected byeither the first or second surfaces 1302 and 1304 is transmitted as beam1316. The two reflected beams 1312 and 1314 overlap and interfere,causing an interference pattern that may be detected by a detector unit.

[0100] The Fresnel etalon 1200 and the binary etalon 1300 may be used indifferent configurations of wavelength locker. For example, the Fresneletalon 1200 or binary etalon 1300 may be used in a wavelength lockerwhere light is first split from the output beam of the laser to form asecond beam that is subsequently incident on the etalon 1200 or 1300.One example of such a configuration is shown in FIG. 3. The Fresneletalon 1200 or binary etalon 1300 may also be placed directly in theoutput beam of the laser, for example as is illustrated in theconfigurations shown in FIGS. 9 and 10.

[0101] Other types of fringe-producing optical elements may be used. Forexample, an etalon may have flat and parallel surfaces and have arefractive index that is not uniform across the etalon. One example ofsuch an etalon is a gradient index (GRIN) lens. The variation inrefractive index, however, need not be radial from an axis, as iscommonly found in a GRIN lens. The refractive index of the non-uniformindex etalon may increase from one side of the etalon to the oppositeside. Furthermore, the profile of the refractive index variation may belinear, parabolic, or may be any suitable function of distance acrossthe etalon that results in a fringe pattern being formed in thereflected light. The refractive index may also vary through the etalon,in a direction along the direction of light propagation through theetalon.

[0102] As noted above, the present invention is applicable to wavelengthlocking of tunable lasers, and is believed to be particularly useful forlocking the wavelength of semiconductor lasers used for opticalcommunications. The present invention should not be considered limitedto the particular examples described above, but rather should beunderstood to cover all aspects of the invention as fairly set out inthe attached claims. Various modifications, equivalent processes, aswell as numerous structures to which the present invention may beapplicable will be readily apparent to those of skill in the art towhich the present invention is directed upon review of the presentspecification. The claims are intended to cover such modifications anddevices.

We claim:
 1. A laser system, comprising: a laser producing a beam ofoutput light; a detector unit; and a fringe-producing optical elementdisposed in the beam of output light to direct a portion of the beam ofoutput light to the detector unit as a second light beam, aninterference pattern being produced in the second light beam by thefringe-producing optical element.
 2. A system as recited in claim 1,wherein the laser is a semiconductor laser.
 3. A system as recited inclaim 1, further comprising a light beam collimator disposed on the beamof output light between the laser and the fringe producing element sothat the output light beam at the fringe-producing element issubstantially collimated.
 4. A system as recited in claim 1, wherein thesecond light beam includes a first component from a first side of thefringe-producing optical element and a second component from a secondside of the fringe-producing optical element, the interference patternbeing produces by interference between the first and second components.5. A system as recited in claim 1, wherein the detector unit has atleast three detector elements illuminated by respective portions of theinterference pattern.
 6. A system as recited in claim 5, wherein therespective portions of the interference pattern correspond to regions ofdifferent phase of the interference pattern.
 7. A system as recited inclaim 6, wherein there are n detector elements, n being greater thantwo, and the regions of different phase of the interference pattern arespaced apart by approximately 2π/n.
 8. A system as recited in claim 1,further comprising a reflector disposed between fringe-producing elementand the detector unit to reflect the second light beam from the fringeproducing element to the detector unit.
 9. A system as recited in claim1, wherein the fringe-producing optical element reflects the secondlight beam to the detector unit.
 10. A system as recited in claim 9,wherein the fringe-producing element is a non-parallel etalon.
 11. Asystem as recited in claim 10, wherein the non-parallel etalon is anon-planar etalon.
 12. A system as recited in claim 10, wherein thenon-parallel etalon is a wedged etalon.
 13. A system as recited in claim10, wherein the non-parallel etalon is a Fresnel etalon.
 14. A system asrecited in claim 10, wherein the non-parallel etalon is a binary etalon.15. A system as recited in claim 9, wherein the fringe-producing elementis a diffractive etalon.
 16. A system as recited in claim 1, wherein thefringe-producing optical element transmits the second light beam to thedetector unit.
 17. A system as recited in claim 16, wherein thefringe-producing optical element is a diffractive etalon.
 18. A systemas recited in claim 1, further comprising an output optical fiber and afocusing unit disposed to focus a remaining portion of the output lightbeam into the optical fiber.
 19. A system as recited in claim 1, furthercomprising a control unit coupled to receive light detection informationfrom the detector unit and to determine an output power of the laser,the control unit further being coupled to the laser to stabilize theoutput power of the laser to a desired power level.
 20. A system asrecited in claim 1, further comprising a control unit coupled to receivelight detection information from the detector unit and to determine ashift of an operating wavelength of the laser from a desired wavelength,the control unit further being coupled to the laser to tune theoperating wavelength of the laser to the desired wavelength.
 21. Asystem as recited in claim 1, wherein the second light beam has a powerlevel of no more than about 10% of a power level of the output lightbeam incident on the fringe-producing element.
 22. An opticalcommunications system, comprising: an optical communications transmitterunit having one or more laser units, at least one of the one or morelaser units producing a laser output beam and having a wavelengthstabilizing unit, the wavelength stabilizing unit including a detectorunit, a fringe-producing optical element disposed in the laser outputbeam to direct a portion of the laser output beam to the detector unitas a second light beam, the fringe-producing optical element causing aninterference pattern in the second light beam, and a control unitcoupled to receive detection signals from the detector unit and adaptedto generate a laser frequency control signal for controlling wavelengthof the at least one of the one or more laser units, an opticalcommunications receiver unit; and an optical fiber communications linkcoupled to transfer optical communications signals from the opticalcommunications transmitter unit to the optical communications receiverunit.
 23. A system as recited in claim 22, further comprising a seriesof fiber amplifiers disposed on the optical fiber communications link,the series of fiber amplifiers including at least one fiber amplifierunit.
 24. A system as recited in claim 22, wherein the opticalcommunications transmission unit includes at least two laser unitsoperating at different wavelengths and further comprising wavelengthdivision multiplexing elements to combine light output from the at leasttwo laser units to produce a multiple channel optical communicationssignal coupled to the optical fiber communications link.
 25. A system asrecited in claim 24, wherein the optical communications receiver unitincludes wavelength division demultiplexing elements to separate themultiple channel optical communications signal into signal components ofdifferent wavelengths and further includes channel detectors to detectrespective signal components.
 26. A method of stabilizing an operatingfrequency of an output light beam produced by a laser, the methodcomprising: deflecting a portion of the output light beam as a secondlight beam using a fringe-producing optical element, thefringe-producing optical element causing an interference fringe patternin the second light beam; detecting portions of the interference fringepattern using a detector unit; producing detector signals in response tothe detected portions of the interference fringe pattern; generating afrequency control signal in response to the detector signals; and tuningthe laser in response to the frequency control signal so that theoperating frequency of the output light beam is substantially at adesired value.
 27. A method as recited in claim 26, wherein deflectingthe portion of the output light beam includes reflecting the portion ofthe output light beam using the fringe-producing optical element.
 28. Amethod as recited in claim 27, wherein the fringe-producing opticalelement is a non-parallel etalon.
 29. A method as recited in claim 28,wherein the non-parallel etalon is a wedged etalon.
 30. A method asrecited in claim 28, wherein the non-parallel etalon is a non-planaretalon.
 31. A method as recited in claim 28, wherein the non-paralleletalon is a Fresnel etalon.
 32. A method as recited in claim 28, whereinthe non-parallel etalon is a binary etalon.
 33. A method as recited inclaim 27, wherein the fringe-producing optical element is a diffractiveetalon.
 34. A method as recited in claim 26, wherein deflecting theportion of the output light beam includes transmitting the portion ofthe output light beam through the fringe-producing optical element. 35.A method as recited in claim 34, wherein the fringe-producing opticalelement is a diffractive etalon.
 36. A method as recited in claim 26,wherein deflecting the portion of the output light beam includesdeflecting no more than about 10% of the output light beam.
 37. A methodas recited in claim 26, wherein deflecting the portion of the outputlight beam includes deflecting a first component from a first side ofthe fringe-producing optical element and deflecting a second componentfrom a second side of the fringe-producing optical element, the secondbeam including the first and second components.
 38. A method as recitedin claim 26, wherein detecting the portions of the interference fringepattern include detecting at least three different portions of theinterference fringe pattern.
 39. A method as recited in claim 38,wherein the at least three different portions of the interference fringepattern correspond to regions of different phase of the interferencepattern.
 40. A method as recited in claim 39, wherein the regions ofdifferent phase of the interference fringe pattern are selected frommore than one period of the interference fringe pattern.
 41. A method asrecited in claim 38, wherein the detector unit has n detector elementsthat detect respective portions of the interference pattern spaced apartby approximately 2π/n.
 42. A method as recited in claim 26, whereingenerating the frequency control signal includes generating phasesignals from the detector signals, and generating transformed phasesignals using the phase signals and reference phase signals.
 43. Amethod of stabilizing an operating frequency of an output light beamproduced by a laser, the method comprising: deflecting a portion of theoutput light beam as a second light beam using a fringe-producingoptical element, the fringe-producing optical element causing aninterference fringe pattern in the second light beam; and stabilizingthe operating frequency of the output light beam using the interferencefringe pattern.
 44. A method as recited in claim 43, wherein deflectingthe portion of the output light beam includes reflecting the portion ofthe output light beam using the fringe-producing optical element.
 45. Amethod as recited in claim 44, wherein the fringe-producing element is anon-parallel etalon.
 46. A method as recited in claim 44, wherein thefringe-producing element is a diffractive etalon.
 47. A method asrecited in claim 43, wherein deflecting the portion of the output lightbeam includes transmitting the portion of the output light beam throughthe fringe-producing optical element.
 48. A method as recited in claim47, wherein the fringe-producing etalon is a diffractive etalon.
 49. Amethod as recited in claim 43, wherein deflecting the portion of theoutput light beam includes deflecting no more than about 10% of theoutput light beam.
 50. A system for stabilizing an operating frequencyof an output light beam produced by a laser, the system comprising:deflecting means for deflecting a portion of the output light beam as asecond light beam, the deflecting means producing an interference fringepattern in the second light beam; and means for stabilizing theoperating frequency of the output light beam using the interferencefringe pattern.